The vast majority of conventional electric motors have stator cores constructed from sheets of laminated steel. The individual laminations are punched from flat sheets of steel using specially constructed dies with the necessary shape of slots and teeth incorporated in them. Laminations made by this method are coated with a thin insulation layer, and then a plurality of these laminations are stacked together to form the complete laminated stator. The construction of the stator core with the laminations separated by layers of very thin insulation is intended to control the iron losses experienced in the stator. These losses are a function of the thickness of the lamination and of the material used. This type of insulation technique could be considered to be on the macroscopic level, implying that bulk volumes of material are insulated from each other by an electrically (but not magnetically) insulating layer. Other parameters affecting the iron losses are also important but are outside the scope of this discussion.
A second but less widely used conventional construction typically involves cold pressing raw metal powder into a "green" shape, followed by sintering the product to improve its mechanical properties. It is well known that this technology produces parts with minimal waste of material having good dimensional tolerance (with very little machining required). It is also an effective method of reliably producing parts with complex geometry. Sintering such green shapes, which involves diffusion between particles of controlled size and properties, is typically accomplished at or less than the melting temperature of the material. Sintering increases mechanical strength, magnetic permeability and, unfortunately, iron losses. The increase in iron losses is so significant that some form of macroscopic insulation technique must be employed to control the phenomenon within acceptable limits. The reason for the increase in iron losses during the sintering process is that diffusion, which occurs between the particles, increases their electrical continuity and eddy current losses.
In order to overcome this problem, Reen et al. (U.S. Pat. No. 4,255,494) cold presses powder metal into laminations having a thickness of between 0.008 and 0.150 inch. These "green" structures are then sintered to increase their mechanical strength. Although these parts form complete annular structures, several of these plate-like structures are stacked on top of each other and fastened together to make a stator. The individual particles are not insulated, however electric insulation is provided (on the macroscopic level) by an insulating layer placed between the laminations.
In another approach, Horie et al. (U.S. Pat. No. 4,719,377) describes the production of a complete stator using powder and a resin in a process of cold compaction. In order to avoid the deterioration of the magnetic characteristics, the finished part is not sintered. The parts made by this method do not possess a high tensile strength. To improve the magnetic characteristics, by achieving a higher permeability and a higher saturation flux-density, inorganic powders are mixed in the resin. To decrease the high-frequency losses, a very small quantity of a coupling agent is added to the mixture before compaction.
Eddy current losses in the powdered iron core are caused by variations in the magnetic field. Magnetic field variations are the result of rotation of a rotor (which has permanent magnets mounted on it), and changes in current passing through the motor windings. It is well established that the reaction of iron powder with phosphoric acid results in an iron phosphate coating on the individual particles which decreases the electrical continuity in the iron and reduces the eddy current losses. Subsequent sintering of these particles would destroy the electrical insulating properties of the phosphate coating.
Fisher et al. (U.S. Pat. No. 5,004,944) assigned to the present assignee, uses flux carrying elements comprised of "green" or cold pressed iron powder containing a phosphate coating. Also disclosed is the use of "B" stage epoxy and wax as binders. Although electromagnetic properties have been acceptable, mechanical properties of the material make it unsuitable for some structural applications. The highest value of tensile strength achievable with cold pressing is about 2,000 lb/in.sup.2. This value is not high enough to practically enable further processing and handling of the product, and is certainly not high enough to withstand the forces required for high power density motors. This shortcoming in mechanical strength is compensated for by encapsulating or impregnating the armature assembly with glass fiber reinforced epoxy, cast as a binding agent between the windings and the iron powder bars. However, rigidity of the structure is dependent on the elastic modulus of the epoxy. Depending on the stator configuration, the relatively low elastic modulus of epoxy, in certain circumstances, has potential for allowing undesirable deformations and dynamic effects within the stator, caused by oscillatory electromagnetic forces.
It has recently become possible, as explained by Rutz et al (U.S. Pat. No. 5,063,011) to use iron powder coated with a thermoplastic material, in addition to the phosphate layer. This product is referred to as double coated iron. Increased mechanical properties are attained by pressing the powder at a temperature sufficiently high to melt the thermoplastic material, but not high enough to allow large scale diffusion between the phosphate coated iron particles. In addition to the benefit of higher tensile strength, the volume of material which can be pressed is significantly larger when using the thermoplastic coated powder. In contrast to iron cores which are produced with a sintering process, the electrical insulating properties that exist in iron cores produced with particles which are double coated can be considered to be microscopic. In other words, each iron particle is adequately insulated from its adjacent particles. Eddy current losses are controlled on a microscopic level, rather than an a macroscopic level.
Ward et al (U.S. Pat. No. 4,947,065) describes an invention in which a complete stator core is produced in one piece by pressing double coated powdered iron in a large mold. While this procedure makes it possible to avoid punching thin individual laminations and then stacking them, this method has significant drawbacks. The initial cost of the die is significantly higher than a die necessary for punched lamination and can only be justified by the prospect of large volume production. In addition, it is difficult accurately to control the properties of the material contained in a part which has complex geometry and large surface area. Another problem of significant magnitude is the need for a very high tonnage press to compact the powder to the required density. From a mechanical standpoint, these limitations make the single piece stator approach impractical in the case of producing stators for high power density motors.
A single-piece core structure also makes it very difficult to implement the high pole count and narrow slot motor concepts used to reduce motor weight as contemplated in Fisher '944. In Fisher '944 it is demonstrated that fine diameter wire in combination with a large number of poles results in a motor with a high power to weight ratio and a high efficiency. This is achieved by distributing the winding over a larger number of slots compared to conventional motors. A reduction is also seen in the eddy currents induced in the wire which is commonly experienced when using large diameter wire. These features require that the stator or armature be designed with a large number of teeth (and slots). The individual teeth are, therefore, required to be narrow to distribute the coils more evenly around the armature, and to accommodate a high pole count in comparison with conventional designs with concentrated windings and fewer poles. The construction of these narrow teeth or flux carrying members has been successfully demonstrated using iron powder technology, and the parts produced have the necessary magnetic characteristics. The design of such an armature also allows for the implementation of various winding patterns. The rotor disclosed in Fisher '944 is of "double ring" construction. The stator core is positioned between the inner and outer rotating rings. Because of this construction the electromagnetic forces acting radially on the core are balanced. Resulting radial deformations of the core are symmetrical, and therefore tolerable for normal operation of the device. However, there are three disadvantages associated with using a rotor which is of the double ring design as compared to a single ring design: (a) the cost of magnets is double because twice as many magnets are used; (b) mass moment of inertia about the spin axis is significantly greater, resulting in lower motor acceleration; and (c) mechanical noise in these rotors is greater, which could be unacceptable in certain applications.
An alternative design for Fisher '944 is a single ring rotor configuration with a stator core comprised of multiple iron powder segments. Radial electromagnetic forces do not act symmetrically on this core. Due to the difference in rotor configuration and the forces acting on the core, it is possible that the core structure disclosed in Fisher '944 is more appropriate for the double ring design because the relatively low elastic modulus of epoxy at high temperature could allow unacceptable large radial deformations of the core in the single ring design.